Lens system and projector utilizing the same

ABSTRACT

A high-performance lens system including lens elements of small diameters for projecting on to a screen or the like in an enlarged fashion images from light valves such as mainly DMDs (Digital Micromirror Devices) for forming images by changing reflecting directions of light is provided. 
     A lens system includes, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole, and depending upon applications, a third lens group made up of a single positive lens element is provided in the vicinity of a light valve such as a DMD on a contracting side of the second lens group.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2006-334017, filed on Dec. 12,2006, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens system including lens elementsof small diameters for projecting on to a screen in an enlarged fashionimages from light valves such as mainly DMDs (Digital MicromirrorDevices) for forming images by changing reflecting directions of light.

2. Description of the Related Art

Projectors utilizing DMDs as light valves are considered as being moreadvantageous in miniaturization than those utilizing other image formingmethods. Currently, compact portable projectors have been well acceptedby many users, and especially so are data projectors which areconvenient for presentations.

In projectors which are developed for use as portable ones, reducing thethickness dimension is of importance, and it can be said that thereduction in thickness is one of the most important factors forprojectors which are in many cases carried together with note-typepersonal computers or the like for combined use. As an example of ameans for solving the problem, for example, the Japanese UnexaminedPatent Publication No. 2003-121736 describes a design method forreducing the effective diameter of a projection lens.

SUMMARY OF THE INVENTION

The present invention has been made in view of the situations and anobjective thereof is to realize a lens system made up of lens elementsof small diameters, so as to provide a projector incorporating the lenssystem so realized which is thus thin enough for convenient carriage andwhich can project an enlarged image with high image quality even in alimited space.

According to a preferred aspect of the invention, there is provided alens system including, sequentially in that order from a magnifyingside, a first lens group which makes up a substantially afocal opticalsystem as a whole and a second lens group having a positive refractivepower as a whole.

The first lens group is configured to include, sequentially in thatorder from a magnifying side, a first sub-lens group or 1a lens grouphaving a negative refractive power as a whole and a second sub-lensgroup or 1b lens group having a positive refractive power as a whole,and the second lens group is configured to include, sequentially in thatorder from a magnifying side, a first sub-lens group or 2a lens grouphaving a negative refractive power as a whole and a second sub-lensgroup or 2b lens group having a positive refractive power as a whole.

In addition, this lens system is such that the following conditionalexpression (1) is satisfied with respect to a power set for the firstlens group, the following conditional expression (2) is satisfied withrespect to an afocal magnification set for the first lens group, thefollowing conditional expression (3) is satisfied with respect to apower set for the 1a lens group, the following conditional expression(4) is satisfied with respect to a power set for the 2a lens group, thefollowing conditional expression (5) is satisfied with respect to apower set for the 2b lens group, and the following conditionalexpression (6) is satisfied with respect to a thickness dimension at anoptical axis of the first lens group.

−0.3≦f/f _(I)≦0.3  (1)

0.35≦h _(IE) /h _(IX)≦0.45  (2)

−1.4≦f/f _(Ia)≦0.8  (3)

−0.5≦f/f _(IIa)≦0.1  (4)

0.35≦f/f _(IIb)≦0.65  (5)

1.9≦T _(I) /f≦2.5  (6)

where,f: Composite focal length of the whole lens system;f_(I): Composite focal length of the first lens group;h_(IE): Height of paraxial ray incident on to a magnifying-side surfaceof a lens element which is disposed outermost on a magnifying side ofthe first lens group;h_(IX): Height of paraxial ray emerging from a contracting-side surfaceof a lens element which is disposed outermost on a contracting side ofthe first lens group;f_(Ia): Composite focal length of the 1a lens group which makes up thefirst lens group;f_(IIa): Composite focal length of the 2a lens group which makes up thesecond lens group;f_(IIb): Composite focal length of the 2b lens group which makes up thesecond lens group; andT_(I): Distance on the optical axis between the magnifying-side surfaceof the lens element which is disposed outermost on the magnifying sideof the first lens group to the contracting-side surface of the lenselement which is disposed outermost on the contracting side of the firstlens group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the configuration of lens elements of a lenssystem according to a first embodiment of the invention,

FIG. 2 is a drawing showing aberrations produced by the lens systemaccording to the first embodiment;

FIG. 3 is a drawing showing the configuration of lens elements of a lenssystem according to a second embodiment of the invention,

FIG. 4 is a drawing showing aberrations produced by the lens systemaccording to the second embodiment;

FIG. 5 is a drawing showing the configuration of lens elements of a lenssystem according to a third embodiment of the invention,

FIG. 6 is a drawing showing aberrations produced by the lens systemaccording to the third embodiment;

FIG. 7 is a drawing showing the configuration of lens elements of a lenssystem according to a fourth embodiment of the invention,

FIG. 8 is a drawing showing aberrations produced by the lens systemaccording to the fourth embodiment;

FIG. 9 is a drawing showing the configuration of lens elements of a lenssystem according to a fifth embodiment of the invention,

FIG. 10 is a drawing showing aberrations produced by the lens systemaccording to the fifth embodiment;

FIG. 11 is a drawing showing the configuration of lens elements of alens system according to a sixth embodiment of the invention,

FIG. 12 is a drawing showing aberrations produced by the lens systemaccording to the sixth embodiment;

FIG. 13 is a drawing showing the configuration of lens elements of alens system according to a seventh embodiment of the invention,

FIG. 14 is a drawing showing aberrations produced by the lens systemaccording to the seventh embodiment;

FIG. 15 is a drawing showing the configuration of lens elements of alens system according to an eighth embodiment of the invention,

FIG. 16 is a drawing showing aberrations produced by the lens systemaccording to the eighth embodiment;

FIG. 17 is a drawing showing an external appearance of a projector whichutilizes the lens system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described by referenceto specific numerical examples. In Embodiments 1 to 8 below, a lenssystem is configured to include, sequentially in that order from amagnifying side, a first lens group LG1 which makes up a substantiallyafocal optical system as a whole and a second lens group LG2 having apositive refractive power as a whole.

The first lens group LG1 is configured to include, sequentially in thatorder from a magnifying side, a first sub-lens group or 1a lens groupLG1 a having a negative refractive power as a whole and a secondsub-lens group or 1b lens group LG1 b having a positive refractive poweras a whole.

The 1a lens group LG1 a is configured to include three lens elementswhich are, sequentially in that order from a magnifying side, a lenselement (lens element designated as L111, a magnifying-side surface as111, a contracting-side surface as 112) which is formed into a meniscusshape which is convex on a magnifying side thereof and which has anegative refractive power (hereinafter, referred to as a negative lenselement), a lens element (lens element designated as L112, amagnifying-side surface as 113, a contracting-side surface as 114) whichhas a positive refractive power (hereinafter, referred to as a positivelens element), and a negative lens element (lens element designated asL113, a magnifying-side surface as 115, a contracting-side surface as116).

The 1b lens group LG1 b, which follows the 1a lens group LG1 a, isconfigured to include four lens elements in total, that is, a negativelens element (lens element designated as L121, a magnifying-side surfaceas 121, a contracting-side surface as 122), a positive lens element(lens element designated as L122, a magnifying-side surface as 123, acontracting-side surface as 124, however, in the event that the lenselement constitutes a cemented lens element together with the precedingnegative lens element, the lens surface 122 and the lens surface 123constitute the same lens surface), a positive lens element (lens elementdesignated as L123, a magnifying-side surface as 124, a contracting-sidesurface as 125), and a positive lens element (lens element designated asL124, a magnifying-side surface as 126, a contracting-side surface as127) which has a meniscus shape which is convex on a contracting sidethereof.

The second lens group LG2 is configured to include, sequentially in thatorder from a magnifying side, a first sub-lens group or 2a lens groupLG2 a having a negative refractive power as a whole and a secondsub-lens group or 2b lens group LG2 b having a positive refractive poweras a whole.

The 2a lens group LG2 a is configured to include two lens elements, thatis, a positive lens element (lens element designated as L211, amagnifying-side surface as 211, a contracting-side surface as 212) and anegative lens element (lens element designated as L212, amagnifying-side surface as 213, a contracting-side surface as 214), orconfigured to include three lens elements in total, that is, a positivelens element (lens element designated as L211, a magnifying-side surfaceas 211, a contracting-side surface as 212), a negative lens element(lens element designated as L212, a magnifying-side surface as 213, acontracting-side surface as 214, however, in the event that the lenselement constitutes a cemented lens element together with the precedingpositive lens element, the lens surface 212 and the lens surface 213constitute the same lens surface), and a positive lens element (lenselement designated as L213, a magnifying-side surface as 215, acontracting-side surface as 216, however, in the event that the lenselement constitutes a cemented lens element together with the precedingnegative lens element, the lens surface 214 and the lens surface 215constitute the same lens surface).

In addition, the 2b lens group LG2 b is configured to include three lenselements in total, that is, a positive lens element (lens elementdesignated as L221, a magnifying-side surface as 221, a contracting-sidesurface as 222), a negative lens element (lens element designated asL222, a magnifying-side surface as 223, a contracting-side surface as224), and a positive lens element (lens element designated as L223, amagnifying-side surface as 225, a contracting-side surface as 226,however, in the event that the lens element constitutes a cemented lenselement together with the preceding negative lens element, the lenssurface 224 and the lens surface 225 constitute the same lens surface).

Additionally, the 1a lens group LG1 a and the 1b lens group LG1 b, whichmake up the first lens group LG1, are fixed to a first lens barrel, andthe 2a lens group LG2 a and the 2b lens group LG2 b, which make up thesecond lens group LG2, are fixed to a second lens barrel.

In addition, depending upon applications, the lens system is configuredto include a third lens group LG3 which is made up of a positive lenselement (lens element designated as L301, a magnifying-side surfacedesignated as 301, a contracting-side surface designated as 302) whichis disposed in the vicinity of a light valve such as a DMD on thecontracting side of the second lens group LG2. A cover glass CG (amagnifying-side surface as C01, a contracting-side surface as C02),which is a constituent component of the light valve such as the DMD, isprovided between a contracting side of the third lens group LG3 and asurface of the light valve with a slight airspace provided therebetween.

In this way, according to the embodiment, there is provided a lenssystem comprising, sequentially in that order from a magnifying-side, afirst lens group which makes up a substantially afocal optical system asa whole and a second lens group having a positive refractive power as awhole, the first lens group being configured to include, sequentially inthat order from a magnifying side, a first sub-lens group or 1a lensgroup having a negative refractive power as a whole and a secondsub-lens group or 1b lens group having a positive refractive power as awhole, the second lens group being configured to include, sequentiallyin that order from a magnifying side, a first sub-lens group or 2a lensgroup having a negative refractive power as a whole and a secondsub-lens group or 2b lens group having a positive refractive power as awhole, wherein, the following conditional expression (1) is satisfiedwith respect to a power set for the first lens group, the followingconditional expression (2) is satisfied with respect to an afocalmagnification set for the first lens group, the following conditionalexpression (3) is satisfied with respect to a power set for the 1a lensgroup, the following conditional expression (4) is satisfied withrespect to a power set for the 2a lens group, the following conditionalexpression (5) is satisfied with respect to a power set for the 2b lensgroup, and the following conditional expression (6) is satisfied withrespect to a thickness dimension at an optical axis of the first lensgroup;

−0.3≦f/f _(I)≦0.3  (1)

0.35≦h _(IE) /h _(IX)≦0.45  (2)

−1.4≦f/f _(Ia)≦0.8  (3)

−0.5≦f/f _(IIa)≦0.1  (4)

0.35≦f/f _(IIb)≦0.65  (5)

1.9≦T _(I) /f≦2.5  (6)

where,f: Composite focal length of the whole lens system;f_(I): Composite focal length of the first lens group;h_(IE): Height of paraxial ray incident on to a magnifying-side surfaceof a lens element which is disposed outermost on a magnifying side ofthe first lens group;h_(IX): Height of paraxial ray emerging from a contracting-side surfaceof a lens element which is disposed outermost on a contracting side ofthe first lens group;f_(Ia): Composite focal length of the 1a lens group which makes up thefirst lens group;f_(IIa): Composite focal length of the 2a lens group which makes up thesecond lens group;f_(IIb): Composite focal length of the 2b lens group which makes up thesecond lens group; andT_(I): Distance on the optical axis between the magnifying-side surfaceof the lens element which is disposed outermost on the magnifying sideof the first lens group to the contracting-side surface of the lenselement which is disposed outermost on the contracting side of the firstlens group.

The conditional expression (1) specifies an appropriate powerdistribution to the first lens group which makes up the substantiallyafocal optical system as a whole. In the event that an upper limit issurpassed, the positive power of the first lens group becomes too large,and the back focus becomes short. In the event that a lower limit issurpassed, the negative power of the first lens group becomes too large,and the load borne by the rear group increases, whereby the aberrationsare deteriorated.

In addition, the conditional expression (2) specifies the magnificationof the first lens group which functions as a wide converter, and in theevent that an upper limit thereof is surpassed, the magnificationbecomes too high, whereby the load in terms of aberrations borne by thefront group becomes too large, resulting in a deteriorated performance,whereas a lower limit is surpassed, the magnification decreases, whichis disadvantageous in wide-angle setting.

The conditional expression (3) is associated with the negative power setfor the 1a lens group which corresponds to a dispersion system in thefirst lens group which makes up the wide converter. In the event that anupper limit is surpassed, the negative power becomes weak, whereby thefocal length of the whole converter system is increased, and the systemhas to be enlarged. In the event that a lower limit is surpassed, thenegative power becomes too large, which is advantageous inminiaturization but is disadvantageous in correcting the aberrations.

The following conditional expressions (4), (5) are associated with thepower configuration of the second lens group. Although the second lensgroup, which makes up the rear group, has the positive power as thewhole system, the second lens group is characterized in that since the2a lens group, which is situated on the magnifying side, constitutes thedispersion system, the 2a lens group is configured to have the negativepower as a whole, and since the 2b lens group, which is situated on thecontracting side, constitutes a condenser system, the 2b lens group isconfigured to have the positive power as a whole.

The conditional expression (4) specifies the negative power of the 2alens group, and in the event that an upper limit is surpassed, thenegative power becomes small, whereby the back focus is shortened. Onthe contrary, in the event that a lower limit is surpassed, the negativepower becomes too large, giving rise to enlargement of the system, andthe aberrations are increased by the negative power which now becomesexcessive.

The conditional expression (5), which follows the expression (4),specifies the positive power of the 2b lens group so as to obtain anappropriate size and performance for the whole lens system together withthe conditional expression (4). In the event that an upper limit issurpassed, the positive power becomes immoderate, and the performance isdecreased and the back focus is shortened. In the event that a lowerlimit is surpassed, the positive power becomes small, giving rise toenlargement of the system.

The conditional expression (6) specifies the thickness of the first lensgroup in the optical axis direction. Functioning as the afocalconverter, the first lens group needs to keep a certain axial thicknessfor correction of the aberrations, although it depends upon the afocalmagnification thereof. In the event that an upper limit is surpassed,although it is advantageous in terms of performance, the system has tobe enlarged, whereas in the event that a lower limit is surpassed,although it is advantageous in terms of miniaturization, the powers ofthe 1a lens group and the 1b lens group become too large, whereby theaberrations are increased.

In addition, the 1a lens group, which makes up the first lens group, isconfigured to include three lens elements, sequentially in that orderfrom a magnifying side, a negative lens element which has a meniscusshape which is convex on a magnifying side thereof, a positive lenselement, and a negative lens element, and it is preferable that thefollowing conditional expression (7) is satisfied with respect to theshape of a contracting-side surface of the lens element which isdisposed outermost on the magnifying side of the 1a lens group, thefollowing conditional expression (8) is satisfied with respect to thedispersion properties of a glass material used for each of the lenselements which make up the 1a lens group, and the following expression(9) is satisfied with respect to the refractive index of a glassmaterial used for each of the lens elements which make up the 1a lensgroup.

0.7≦R _(Ia2) /f≦1.3  (7)

10≦V _(IaN) −V _(Iap)  (8)

1.7≦N_(Ia)  (9)

where,

R_(Ia2): Radius of curvature of the contracting-side surface of the lenselement which is disposed outermost on the magnifying side of the 1alens group; V_(IaN): Mean value of Abbe numbers of the negative lenselement which makes up the 1a lens group; V_(IaP): Mean value of Abbenumbers of the positive lens element which makes up the 1a lens group;and N_(Ia): Mean value of refractive indices of the lens elements whichmake up the 1a lens group relative to the d line.

The conditional expression (7) is associated with the shape of thecontracting-side surface of the lens element which is disposed outermoston the magnifying side of the 1a lens group, and the contracting-sidesurface is formed substantially into a concentric shape relative to abundle of rays on the magnifying side while being made to have a strongpower, and basically, the concentric shape so formed is made to suppressthe generation of aberrations. Consequently, in the event that a lowerlimit is surpassed, spherical aberration and comatic aberration areovercorrected, whereas in the event that an upper limit is surpassed,they become undercorrected, on the contrary.

The conditional expression (8) is an achromatic condition whichconstitutes a basic factor of the first lens group and is a conditionalexpression for basically decreasing the quantity of chromatic aberrationto be generated and maintaining a proper achromatic condition. Namely,the conditional expression expresses the balance of dispersionproperties of a glass material used for the 1a lens group which bears alarge negative power in the first lens group having a strong negativepower, and it is important to suppress the basic chromatic aberrationbeing generated in the interior of the lens group to a small level bysatisfying this conditional expression. By selecting glass materials forthese lens elements under the condition specified by the conditionalexpression (8), a suitable power distribution can be realized, and thechromatic aberration can be corrected properly. In the event that alower limit is surpassed, the power of each lens element becomesexcessive for correction of the chromatic aberration, whereby theaberrations are worsened.

The conditional expression (9) is associated with the characteristics ofthe refractive index of the 1a lens group which has the particularlystrong negative power. In order to mitigate the intensity of curvatureof acquiring the strong negative power, it is inevitable that therefractive index is as high as possible, and in the event that a lowerlimit of the conditional expression is surpassed, the curvature becomesexcessive, whereby the spherical aberration and the comatic aberrationbecome excessive, and the Petzval sum becomes too small, thereby makingit impossible to obtain a good performance.

In addition, the 1b lens group is configured to include four lenselements in total, that is, a negative lens element, a positive lenselement, a positive lens element and a positive lens element which has ameniscus shape which is convex on a contracting side thereof, and it ispreferable that the following conditional expression (10) is satisfiedwith respect to the shape of a contracting-side surface of the lenselement which is disposed fourth outermost from the magnifying side andthe following conditional expression (11) is satisfied with respect tothe dispersion properties of a glass material used for each lens elementwhich makes up a cemented lens disposed on the magnifying side.

−1.5≦R _(Ib7) /f≦−0.9  (10)

10≦V _(IbP) −V _(IbN)  (11)

where,

R_(Ib7): Radius of curvature of the contracting-side surface of the lenselement which is disposed fourth outermost from the magnifying side ofthe 1b lens group; V_(IbP): Mean value of Abbe numbers of the positivelens elements which make up the 1b lens group; and V_(IbN): Mean valueof Abbe numbers of the negative lens element which makes up the 1b lensgroup.

The conditional expression (10) is associated with the shape of the lenselement which is disposed fourth outermost from the magnifying side ofthe 1b lens group, that is, the lens element which is disposed outermoston the contracting side, and the shape has a role of transmitting abundle of dispersed rays from the 1a lens group to the rear whilemaintaining the generation of aberrations to a low level. In the eventthat an upper limit is surpassed, the correction of a large negativedistortion produced by the 1b lens group becomes insufficient, whereasin the event that a lower limit is surpassed, although it isadvantageous in correcting the distortion, the meniscus shape becomestoo intensive, and high-order spherical aberration and comaticaberration are generated.

The conditional expression (11) is associated with the achromatism ofthe 1b lens group and specifies an appropriate combination of Abbenumbers in the glass material. Consequently, in a condition where alower limit is surpassed, it becomes difficult to correct chromaticaberration.

In addition, the 2a lens group is configured to include two lenselements, that is, a positive lens element and a negative lens elementor to include three lens elements in total, that is, a positive lenselement, a negative lens element and a positive lens element, and it ispreferable that the following conditional expressions (12) is satisfiedwith respect to a refractive index of a glass material used for each ofthe lens elements which make up the 2a lens group, the followingconditional expression (13) is satisfied with respect to the dispersionproperties of the positive lens elements which is disposed outermost onthe magnifying side of the 2a lens group and the negative lens elementwhich is disposed second outermost from the magnifying side of the 2alens group, the following conditional expression (14) is satisfied withrespect to a power set for the lens element which is disposed outermoston the magnifying side of the 2a lens group and the followingconditional expression (15) is satisfied with respect to the shape of acontracting-side surface of the lens element which is disposed outermoston the magnifying side of the 2a lens group and the shape of amagnifying-side surface of the lens element which is disposed secondoutermost from the magnifying side of the 2a lens group.

0.1≦N _(IIaN) −N _(IIaP)  (12)

10≦V _(IIa1) −V _(IIa2)  (13)

0.2≦f/f _(IIa1)≦0.5  (14)

0.8≦R _(IIa2) /R _(IIa3)≦1.5  (15)

where,

N_(IIaN): Mean value of refractive indices of the negative lens elementwhich makes up the 2a lens group relative to the d line; N_(IIaP): Meanvalue of refractive indices of the positive lens elements which make upthe 2a lens group relative to the d line; V_(IIa1): Abbe number of thelens element which is disposed outermost on the magnifying side of thelens elements which make up the 2a lens group; V_(IIa2): Abbe number ofthe lens element which is disposed second outermost from the magnifyingside of the lens elements which make up the 2a lens group;

f_(IIa1): Focal length of the lens element which is disposed outermoston the magnifying side of the lens elements which make up the 2a lensgroup;

R_(IIa2): Radius of curvature of a contracting-side surface of the lenselement which is disposed outermost on the magnifying side of the lenselements which make up the 2a lens group; and R_(IIa3): Radius ofcurvature of a magnifying-side surface of the lens element which isdisposed second outermost from the magnifying side of the lens elementswhich make up the 2a lens group.

The conditional expression (12) specifies a condition for fieldcurvature correction, which corrects a field curvature produced by thestrong negative power of the 2a lens group. Namely, it is good to use amaterial having a high refractive index for the negative lens elementand a material having a low refractive index for the positive lenselement, so as to prevent the Petzval sum from becoming too small. Inthe event that a lower limit is surpassed, the Petzval sum becomes toosmall, whereby the field curvature is worsened.

The conditional expression (13) is a condition for correction ofchromatic aberration of the 2a lens group. In the event that an upperlimit is surpassed, the Abbe numbers come nearer to each other, and thechromatic aberration effect becomes insufficient.

The conditional expression (14) is, as with the conditional expression(13), a condition for correction of chromatic aberration of the 2a lensgroup. As is known, since chromatic aberration can be corrected by anappropriate combination of Abbe number and power, the conditionalexpression (14) specifies the power. In the event that an upper limitand a lower limit are surpassed, in both cases, the achromatic power ofthe 2a lens group becomes inappropriate, whereby the chromaticaberration are worsened.

The conditional expression (15) specifies a restricting conditionregarding the shape of the 2a lens group and is associated withcorrection of spherical aberration and comatic aberration. In the eventthat an upper limit is surpassed, the spherical aberration becomes over,and the comatic aberration relative to a bundle of off-axis rays is alsoworsened. On the contrary, in the event that a lower limit is surpassed,the spherical aberration becomes under excessively.

In addition, the 2b lens group is configured to include three lenselements in total, that is, a positive lens element, a negative lenselement, and a positive lens element, and it is preferable that thefollowing conditional expressions (16), (17) are satisfied with respectto refractive index and dispersion properties of a glass material usedfor each of the lens elements which make up the 2b lens group,respectively, the following conditional expression (18) is satisfiedwith respect to a power set for the lens element which is disposedsecond outermost from the magnifying side of the 2b lens group, and thefollowing conditional expression (19) is satisfied with respect to theshape of a magnifying-side surface and the shape of a contracting-sidesurface of the lens element which is disposed outermost on themagnifying side of the 2b lens group.

0.15≦N _(IIbN) −N _(IIbP)  (16)

20≦V _(IIbP) −V _(IIbN)  (17)

−0.75≦f/f _(IIb2)≦−0.25  (18)

−6.0≦R _(IIb1) /R _(IIb2)≦−0.8  (19)

where,

N_(IIbN): Mean value of refractive indices of the negative lens elementwhich makes up the 2b lens group relative to the d line; N_(IIbP): Meanvalue of refractive indices of the positive lens elements which make upthe 2b lens group relative to the d line; V_(IIbP): Mean value of Abbenumbers of the positive lens elements which make up the 2b lens group;V_(IIbN): Mean value of Abbe numbers of the negative lens element whichmakes up the 2b lens group;

f_(IIb2): Focal length of the lens element which lies second outermostfrom the magnifying side of the lens elements which make up the 2b lensgroup;

R_(IIb1): Radius of curvature of a magnifying-side surface of the lenselement which is disposed outermost on the magnifying side of the 2blens group; and R_(IIb2): Radius of curvature of a contracting-sidesurface of the lens element which is disposed outermost on themagnifying side of the 2b lens group.

The conditional expression (16) is associated with the refractive indexof a glass material for each of the lens elements which make up the 2blens group which makes up the second lens group and has a large positivepower. By giving a difference in refractive index between the positiveand negative lens elements, a correcting capability of sphericalaberration is made use of on a cemented surface while maintaining thegeneration of chromatic aberration to a minimum level, and the effect offield curvature correction is also expected. In the event that a lowerlimit is surpassed in the conditional expression (16), it results inovercorrection of field curvature and results in undercorrection ofspherical aberration.

The conditional expression (17) specifies a condition for colorcorrection in the 2b lens group, and in order to correct single-coloraberrations at the same time, it is necessary that the powers of therespective lens elements do not become excessive, and to make thishappen, it is a necessary condition that Abbe numbers of glass materialsused for the positive lens elements and the negative lens elementsatisfy the conditional expression (17). In the event that a lower limitis surpassed, it becomes difficult to correct chromatic aberration.

The conditional expression (18) similarly specifies a condition forchromatic aberration correction or achromatism of the 2b lens group. Inthe event that both an upper limit and a lower limit are surpassed, inboth cases, the achromatic power in the 2b lens group becomesinappropriate, and the chromatic aberration is worsened.

The conditional expression (19) specifies a restricting condition forthe shape of the lens element which is disposed outermost on themagnifying side of the 2b lens group and is associated with sphericalaberration and comatic aberration. In the event that an upper limit issurpassed, the spherical aberration becomes under largely, whereas inthe event that a lower limit is surpassed, the generation of comaticflare in a bundle of marginal rays becomes remarkable.

In addition, as a method for adjusting the focal point of the whole lenssystem, a method for shifting the whole projection lens in the opticalaxis direction cannot be said to be a good method in a case where thereis imposed a limitation on the size of a structure to be shifted. Then,in adopting a so-called inner focusing method in which the focal pointis adjusted by shifting part of an optical system of a lens system, whentaking it into consideration that the first lens group is substantiallyafocal, it is considered to make use of the lens group which is disposedcloser to the contracting side than the first lens group, and it ispreferable to adjust the focal point by shifting the second lens groupin the optical axis direction because a spatial amount for shifting issufficient and the change in aberrations due to distance can besuppressed to a least level.

Furthermore, in order to focus bundles of rays from the DMDs on to thesurface of a screen with good efficiency, the angle of a principal rayof a bundle of rays emerging from the DMD must be set according to theproperties of the illuminating optical system. Due to this, the lenssystem as being a projection lens is set substantially telecentric inmany cases. However, when attempting to design such that all the raysare captured, the effective diameter of the second lens group on thecontracting side thereof becomes gigantic, which in many cases causes aproblem with the arrangement of the second lens group relative to theilluminating optical system. As this occurs, although there is imposed alimitation on the effective diameter in the vicinity of the emergingside of the second lens group depending upon the arrangement of thesecond lens group with the illuminating optical system, the objective ispreferably attained by providing a third lens group which is made up ofa single positive lens in the vicinity of the light valve in theairspace between the second lens group and the light valve.

In addition, FIG. 17 shows an external appearance of a projector whichutilizes the lens system of the invention. As is shown in FIG. 17, aprojector 10 is formed substantially into a rectangular parallelepipedshape, and the projector 10 has a lens cover 19 which covers aprojection opening provided at an end of a front plate 12 which makes upa main body housing, and a plurality of vent holes 18 are provided inthe front plate 12. In addition, although the illustration thereof isomitted in FIG. 17, a key/indicator section is provided on an upperplate 11 which also makes up the main body housing, and thiskey/indicator section includes keys and indicators such as a powersupply switch, a power indicator lamp which informs that the powersupply is on or off, a lamp switch key for turning on a lamp of a lightsource unit, an indicator lamp which indicates that the lamp is turnedon and an overheat indicator lamp which informs that the light sourceunit is under overheat when it actually happens. Furthermore, providedon a back plate, which is not shown, of the main body housing are aninput/output connector section where USB terminals, D-SUB terminals forimage signal input, S terminals, RCA terminals and the like are providedand an Ir receiving section where control signals from a power supplyadapter plug and a remote controller are received. In addition, aplurality of vent holes 18 are provided in a right-hand side plate, notshown, which is a side plate of the main body housing, and a left-handside plate 15, which is a side plate shown in FIG. 17, and a projectionlens which adopts the lens system that has been described heretofore isinstalled inside the left-hand side plate 15 therealong.

Embodiment 1

A numerical example of a first embodiment of a lens system of theinvention is shown in Table 1. In addition, FIG. 1 is a drawing showingthe configuration of the lens system, and FIG. 2 is a drawing showingaberrations thereof. In the following tables and drawings, f denotes afocal length of the whole lens system, F_(no) an F number, and 2ω atotal angle of view of the lens system. In addition, r denotes a radiusof curvature, d a lens element thickness or spacing between lenselements, n_(d) a refractive index relative to the d line, and v_(d)Abbe number at the d line. In the drawings showing spherical aberrationof the drawings showing the aberrations, CA1, CA2, CA3 denote aberrationcurves at wavelengths of CA1=550.0 nm, CA2=435.8 nm and CA3=640.0 nm,and S.C denotes a sine condition. In the drawing showing astigmatism, Sdenotes sagital, and M denotes meridional. In addition, throughout thetables, unless otherwise described, the wavelength used in calculationof numerical values is CA1=550.0 nm, and the drawings showing theaberrations show aberrations which result with an object distance of1700 mm which is used in many cases for evaluation of a projection lens.

TABLE 1 f 16.01 F_(no) 2.29 2ω 65.77 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 26.626 1.26 1.83400 37.34 2 112 14.328 5.49 — — 3 113−3101.631 2.72 1.80518 25.46 4 114 −39.554 0.10 — — 5 115 −151.432 1.201.77250 49.62 6 116 16.692 4.78 — — 7 121 −30.843 1.98 1.80518 25.46 8122(123) 25.148 6.90 1.62004 36.30 9 124 −28.414 0.20 — — 10 125 374.6903.70 1.80518 25.46 11 126 −41.814 3.31 — — 12 127 −19.502 3.45 1.4970081.61 13 128 −17.172 38.25 — — 14 211 −97.056 3.77 1.49700 81.61 15 212−20.546 2.34 — — 16 213 −17.176 1.42 1.77250 49.62 17 214 −34.956 0.22 —— 18 221 108.474 5.11 1.49700 81.61 19 222 −25.737 0.20 — — 20 22336.340 1.20 1.83400 37.34 21 224(225) 16.707 7.01 1.48749 70.45 22 226−499.119 34.00 — — 23 301 −100.000 3.00 1.77250 49.62 24 302 −44.0000.63 — — 25 C01 ∞ 3.00 1.48745 64.84 26 C02 ∞ — — —

Embodiment 2

A numerical example of a second embodiment of a lens system of theinvention is shown in Table 2. In addition, FIG. 3 is a drawing showingthe configuration of the lens system, and FIG. 4 is a drawing showingaberrations thereof.

TABLE 2 f 15.95 F_(no) 2.29 2ω 65.96 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 29.623 1.20 1.83400 37.34 2 112 15.130 4.79 — — 3 113169.840 2.77 1.80518 25.46 4 114 −50.405 0.20 — — 5 115 −3436.423 1.201.80420 46.50 6 116 14.444 5.13 — — 7 121 −27.012 1.40 1.80518 25.46 8122(123) 17.852 7.98 1.74950 35.04 9 124 −31.098 1.06 — — 10 125 268.5323.94 1.80518 25.46 11 126 −39.794 3.03 — — 12 127 −21.097 3.41 1.4970081.61 13 128 −17.958 28.17 — — 14 211 −92.353 4.59 1.51680 64.20 15 212−21.593 2.54 — — 16 213 −16.555 1.01 1.80610 40.73 17 214(215) 22.6316.10 1.60342 38.00 18 216 −29.459 0.20 — — 19 221 43.811 6.41 1.4970081.61 20 222 −23.607 0.21 — — 21 223 42.444 1.20 1.83400 37.34 22224(225) 16.494 7.29 1.48749 70.45 23 226 −145.343 34.00 — — 24 301−100.000 3.00 1.77250 49.62 25 302 −44.000 0.63 — — 26 C01 ∞ 3.001.48745 64.84 27 C02 ∞ — — —

Embodiment 3

A numerical example of a third embodiment of a lens system of theinvention is shown in Table 3. In addition, FIG. 5 is a drawing showingthe configuration of the lens system, and FIG. 6 is a drawing showingaberrations thereof.

TABLE 3 f 15.98 F_(no) 2.29 2ω 65.25 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 32.325 1.20 1.83400 37.34 2 112 15.076 5.50 — — 3 113−169.279 2.47 1.80518 25.46 4 114 −38.056 0.20 — — 5 115 231394.363 1.201.77250 49.62 6 116 15.501 4.83 — — 7 121 −33.059 1.40 1.80518 25.46 8122(123) 18.494 7.75 1.74950 35.04 9 124 −32.396 0.60 — — 10 125 185.5173.87 1.80518 25.46 11 126 −43.625 2.88 — — 12 127 −22.063 3.63 1.4874970.45 13 128 −17.956 26.98 — — 14 211 −49.144 4.00 1.51680 64.20 15 212−18.736 1.83 — — 16 213 −15.306 1.00 1.80610 40.73 17 214(215) 21.6796.29 1.60342 38.00 18 216 −27.234 0.20 — — 19 221 43.767 6.57 1.4970081.61 20 222 −22.619 0.20 — — 21 223 40.180 1.20 1.83400 37.34 22224(225) 15.932 7.47 1.48749 70.45 23 226 −137.785 34.00 — — 24 301−100.000 3.00 1.77250 49.62 25 302 −44.000 0.63 — — 26 C01 ∞ 3.001.48745 64.84 27 C02 ∞ — — —

Embodiment 4

A numerical example of a forth embodiment of a lens system of theinvention is shown in Table 4. In addition, FIG. 7 is a drawing showingthe configuration of the lens system, and FIG. 8 is a drawing showingaberrations thereof.

TABLE 4 f 15.95 F_(no) 2.29 2ω 65.97 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 31.347 1.20 1.83400 37.34 2 112 15.210 4.78 — — 3 113191.532 3.01 1.80518 25.46 4 114 −41.224 0.20 — — 5 115 −95.223 1.201.77250 49.62 6 116 14.940 4.85 — — 7 121 −32.233 1.40 1.80518 25.46 8122(123) 19.101 7.67 1.72342 37.99 9 124 −33.224 0.36 — — 10 125 599.8793.94 1.80518 25.46 11 126 −36.665 4.33 — — 12 127 −20.869 3.60 1.5168064.20 13 128 −17.536 29.28 — — 14 211 −211.059 3.37 1.51680 64.20 15 212−20.603 2.28 — — 16 213 −16.124 1.00 1.80610 40.73 17 214(215) 21.0686.22 1.59551 39.23 18 216 −30.035 0.20 — — 19 221 42.487 6.64 1.4970081.61 20 222 −22.539 0.20 — — 21 223 49.857 1.20 1.80610 40.73 22224(225) 15.643 7.32 1.51680 64.20 23 226 −223.858 34.00 — — 24 301−100.000 3.00 1.77250 49.62 25 302 −44.000 0.63 — — 26 C01 ∞ 3.001.48745 64.84 27 C02 ∞ — — —

Embodiment 5

A numerical example of a fifth embodiment of a lens system of theinvention is shown in Table 5. In addition, FIG. 9 is a drawing showingthe configuration of the lens system, and FIG. 10 is a drawing showingaberrations thereof.

TABLE 5 f 15.97 F_(no) 2.29 2ω 65.91 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 32.106 1.50 1.83400 37.34 2 112 14.894 4.44 — — 3 11395.965 3.05 1.80518 25.46 4 114 −48.411 0.20 — — 5 115 −1996.799 2.011.77250 49.62 6 116 13.641 5.39 — — 7 121 −21.536 1.40 1.80518 25.46 8122(123) 18.684 8.09 1.72342 37.99 9 124 −26.697 0.20 — — 10 125 251.8734.06 1.80518 25.46 11 126 −37.708 2.92 — — 12 127 −20.561 3.50 1.4874970.45 13 128 −17.522 27.15 — — 14 211 −172.734 7.81 1.48749 70.45 15 212−19.778 1.09 — — 16 213 −17.414 1.00 1.80610 40.73 17 214(215) 23.0806.02 1.60342 38.00 18 216 −32.704 0.20 — — 19 221 39.122 6.31 1.4874970.45 20 222 −25.803 0.20 — — 21 223 43.911 1.20 1.83400 37.34 22224(225) 16.217 7.23 1.48749 70.45 23 226 −169.264 34.00 — — 24 301−100.000 3.00 1.77250 49.62 25 302 −44.000 0.63 — — 26 C01 ∞ 3.001.48745 64.84 27 C02 ∞ — — —

Embodiment 6

A numerical example of a sixth embodiment of a lens system of theinvention is shown in Table 6. In addition, FIG. 11 is a drawing showingthe configuration of the lens system, and FIG. 12 is a drawing showingaberrations thereof.

TABLE 6 f 15.97 F_(no) 2.29 2ω 65.89 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 32.232 1.20 1.83400 37.34 2 112 15.426 4.38 — — 3 11397.820 2.96 1.80518 25.46 4 114 −54.070 0.20 — — 5 115 229.321 1.241.77250 49.62 6 116 13.450 5.24 — — 7 121 −24.982 2.46 1.80518 25.46 8122(123) 17.389 8.50 1.72342 37.99 9 124 −32.334 0.20 — — 10 125 140.1724.23 1.80518 25.46 11 126 −39.457 2.90 — — 12 127 −21.673 3.46 1.4874970.45 13 128 −18.133 25.41 — — 14 211 −52.526 7.96 1.48749 70.45 15212(213) −15.651 2.59 1.80420 46.50 16 214(215) 25.512 6.15 1.6237447.05 17 216 −28.648 0.20 — — 18 221 34.154 5.96 1.48749 70.45 19 222−32.542 0.20 — — 20 223 41.205 1.20 1.83400 37.34 21 224(225) 15.7977.17 1.48749 70.45 22 226 −320.015 34.00 — — 23 301 −100.000 3.001.77250 49.62 24 302 −44.000 0.63 — — 25 C01 ∞ 3.00 1.48745 64.84 26 C02∞ — — —

Embodiment 7

A numerical example of a seventh embodiment of a lens system of theinvention is shown in Table 7. In addition, FIG. 13 is a drawing showingthe configuration of the lens system, and FIG. 14 is a drawing showingaberrations thereof.

TABLE 7 f 15.94 F_(no) 2.29 2ω 65.99 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 29.379 1.20 1.80100 34.97 2 112 14.845 5.52 — — 3 113−225.975 2.44 1.80518 25.46 4 114 −40.518 0.20 — — 5 115 −117.999 1.201.80420 46.49 6 116 17.476 4.12 — — 7 121 −48.537 1.40 1.78472 25.72 8122(123) 16.648 8.30 1.72047 34.71 9 124 −41.938 2.37 — — 10 125 252.6684.09 1.76182 26.61 11 126 −38.078 2.59 — — 12 127 −22.121 3.33 1.4874970.45 13 128 −18.625 27.28 — — 14 211 −204.975 7.66 1.51680 64.20 15 212−20.634 1.59 — — 16 213 −17.025 1.00 1.80610 40.73 17 214(215) 21.7975.77 1.60342 38.00 18 216 −36.775 0.75 — — 19 221 43.917 6.36 1.4970081.61 20 222 −23.890 0.20 — — 21 223 39.854 1.20 1.83400 37.34 22224(225) 16.602 7.30 1.48749 70.45 23 226 −142.188 34.63 — — 24 C01 ∞3.00 1.48745 64.84 25 C02 ∞ — — —

Embodiment 8

A numerical example of an eighth embodiment of a lens system of theinvention is shown in Table 8. In addition, FIG. 15 is a drawing showingthe configuration of the lens system, and FIG. 16 is a drawing showingaberrations thereof.

TABLE 8 f 15.93 F_(no) 2.29 2ω 66.02 Serial Surface numbers numbers r dn_(d) ν_(d) 1 111 35.726 1.40 1.80610 33.27 2 112 15.882 4.44 — — 3 113182.859 2.52 1.80518 25.46 4 114 −61.465 0.20 — — 5 115 187.732 1.201.80420 46.49 6 116 15.304 5.03 — — 7 121 −27.411 2.17 1.80518 25.46 8122(123) 18.828 8.50 1.76182 26.61 9 124 −28.142 0.20 — — 10 125 462.2423.15 1.80100 34.97 11 126 −54.392 4.82 — — 12 127 −23.852 4.45 1.4874970.45 13 128 −18.669 27.33 — — 14 211 −35.741 2.64 1.71300 53.93 15 212−18.527 1.64 — — 16 213 −15.366 1.00 1.80610 40.73 17 214(215) 24.8895.59 1.59551 39.23 18 216 −33.161 0.20 — — 19 221 54.524 6.29 1.4970081.61 20 222 −22.285 0.20 — — 21 223 51.035 1.20 1.80610 40.73 22224(225) 19.166 6.27 1.48749 70.45 23 226 −197.696 34.00 — — 24 30135.000 20.00 1.51680 64.20 25 302 ∞ 0.63 — — 26 C01 ∞ 3.00 1.48745 64.8427 C02 ∞ — — —

Next, values corresponding to the conditional expression (1) toconditional expression (19) in the first embodiment to the eighthembodiment are shown in Table 9 altogether.

TABLE 9 Conditional Embodiment Expression 1 2 3 4 5 6 7 8  (1) −0.120.03 0.07 0.00 0.04 0.10 0.03 0.03  (2) 0.41 0.40 0.41 0.40 0.39 0.400.41 0.41  (3) −1.00 −1.06 −1.10 −1.11 −1.05 −1.00 −1.10 −1.00  (4)−0.04 −0.23 −0.29 −0.20 −0.19 −0.18 −0.23 −0.33  (5) 0.44 0.53 0.55 0.520.50 0.46 0.55 0.51  (6) 2.19 2.26 2.22 2.29 2.30 2.31 2.31 2.39  (7)0.89 0.95 0.94 0.95 0.93 0.97 0.93 1.00  (8) 18.02 16.46 18.02 18.0218.02 18.02 15.27 14.42  (9) 1.80 1.81 1.80 1.80 1.80 1.80 1.80 1.81(10) −1.07 −1.13 −1.12 −1.10 −1.10 −1.14 −1.17 −1.17 (11) 22.33 21.9118.19 17.09 19.17 19.17 18.20 18.55 (12) 0.28 0.25 0.25 0.25 0.26 0.250.25 0.15 (13) 31.99 23.47 23.47 23.47 29.72 23.95 23.47 13.20 (14) 0.310.30 0.29 0.36 0.36 0.38 0.37 0.32 (15) 1.20 1.30 1.22 1.28 1.14 1.001.21 1.21 (16) 0.34 0.34 0.34 0.30 0.35 0.35 0.34 0.31 (17) 38.69 38.6938.69 32.18 33.11 33.11 38.69 35.30 (18) −0.42 −0.49 −0.50 −0.56 −0.51−0.51 −0.46 −0.41 (19) −4.21 −1.86 −1.93 −1.89 −1.52 −1.05 −1.84 −2.45

As is obvious from Table 9, the numerical values related to therespective examples of the first to eighth embodiments satisfy theconditional expressions (1) to (19), and as is obvious from theaberration drawings of the respective embodiments, the respectiveaberrations were corrected properly.

1. A lens system comprising, sequentially in that order from amagnifying side, a first lens group which makes up a substantiallyafocal optical system as a whole and a second lens group having apositive refractive power as a whole, the first lens group beingconfigured to comprise, sequentially in that order from a magnifyingside, a first sub-lens group or 1a lens group having a negativerefractive power as a whole and a second sub-lens group or 1b lens grouphaving a positive refractive power as a whole, the second lens groupbeing configured to comprise, sequentially in that order from amagnifying side, a first sub-lens group or 2a lens group having anegative refractive power as a whole and a second sub-lens group or 2blens group having a positive refractive power as a whole, wherein, thefollowing conditional expression (1) is satisfied with respect to apower set for the first lens group, the following conditional expression(2) is satisfied with respect to an afocal magnification set for thefirst lens group, the following conditional expression (3) is satisfiedwith respect to a power set for the 1a lens group, the followingconditional expression (4) is satisfied with respect to a power set forthe 2a lens group, the following conditional expression (5) is satisfiedwith respect to a power set for the 2b lens group, and the followingconditional expression (6) is satisfied with respect to a thicknessdimension at an optical axis of the first lens group;−0.3≦f/f _(I)≦0.3  (1)0.35≦h _(IE) /h _(IX)≦0.45  (2)−1.4≦f/f _(Ia)≦0.8  (3)−0.5≦f/f _(IIa)≦0.1  (4)0.35≦f/f _(IIb)≦0.65  (5)1.9≦T _(I) /f≦2.5  (6) where, f: Composite focal length of the wholelens system; f_(I): Composite focal length of the first lens group;h_(IE): Height of paraxial ray incident on to a magnifying-side surfaceof a lens element which is disposed outermost on a magnifying side ofthe first lens group; h_(IX): Height of paraxial ray emerging from acontracting-side surface of a lens element which is disposed outermoston a contracting side of the first lens group; f_(Ia): Composite focallength of the 1a lens group which makes up the first lens group;f_(IIa): Composite focal length of the 2a lens group which makes up thesecond lens group; f_(IIb): Composite focal length of the 2b lens groupwhich makes up the second lens group; and T_(I): Distance on the opticalaxis between the magnifying-side surface of the lens element which isdisposed outermost on the magnifying side of the first lens group to thecontracting-side surface of the lens element which is disposed outermoston the contracting side of the first lens group.
 2. A lens system as setforth in claim 1, wherein the 1a lens group, which makes up the firstlens group, is configured to comprises three lens elements, sequentiallyin that order from a magnifying side, a lens element which is formedinto a meniscus shape which is convex on a magnifying side thereof andwhich has a negative refractive power (hereinafter, referred to as anegative lens element), a lens element which has a positive refractivepower (hereinafter, referred to as a positive lens element), and anegative lens element, and wherein the following conditional expression(7) is satisfied with respect to the shape of a contracting-side surfaceof the lens element which is disposed outermost on the magnifying sideof the 1a lens group, the following conditional expression (8) issatisfied with respect to the dispersion properties of a glass materialused for each of the lens elements which make up the 1a lens group, andthe following expression (9) is satisfied with respect to the refractiveindex of a glass material used for each of the lens elements which makeup the 1a lens group;0.7≦R _(Ia2) /f≦1.3  (7)10≦V _(IaN) −V _(Iap)  (8)1.7≦N_(Ia)  (9) where, R_(Ia2): Radius of curvature of thecontracting-side surface of the lens element which is disposed outermoston the magnifying side of the 1a lens group; V_(IaN): Mean value of Abbenumbers of the negative lens element which makes up the 1a lens group;V_(IaP): Mean value of Abbe numbers of the positive lens element whichmakes up the 1a lens group; and N_(Ia): Mean value of refractive indicesof the lens elements which make up the 1a lens group relative to the dline.
 3. A lens system as set forth in claim 1, wherein the 1b lensgroup is configured to comprises four lens elements in total, that is, anegative lens element, a positive lens element, a positive lens elementand a positive lens element which has a meniscus shape which is convexon a contracting side thereof, and wherein the following conditionalexpression (10) is satisfied with respect to the shape of acontracting-side surface of the lens element which is disposed fourthoutermost from the magnifying side, and the following conditionalexpression (11) is satisfied with respect to the dispersion propertiesof a glass material used for each lens element which makes up a cementedlens disposed on the magnifying side;−1.5≦R _(Ib7) /f≦−0.9  (10)10≦V _(IbP) −V _(IbN)  (11) where, R_(Ib7): Radius of curvature of thecontracting-side surface of the lens element which is disposed fourthoutermost from the magnifying side of the 1b lens group; V_(IbP): Meanvalue of Abbe numbers of the positive lens elements which make up the 1blens group; and V_(IbN): Mean value of Abbe numbers of the negative lenselement which makes up the 1b lens group.
 4. A lens system as set forthin claim 1, wherein the 2a lens group is configured to comprise two lenselements, that is, a positive lens element and a negative lens elementor to comprise three lens elements in total, that is, a positive lenselement, a negative lens element and a positive lens element, andwherein the following conditional expressions (12) is satisfied withrespect to a refractive index of a glass material used for each of thelens elements which make up the 2a lens group, the following conditionalexpression (13) is satisfied with respect to the dispersion propertiesof the positive lens elements which is disposed outermost on themagnifying side of the 2a lens group and the negative lens element whichis disposed second outermost from the magnifying side of the 2a lensgroup, the following conditional expression (14) is satisfied withrespect to a power set for the lens element which is disposed outermoston the magnifying side of the 2a lens group and the followingconditional expression (15) is satisfied with respect to the shape of acontracting-side surface of the lens element which is disposed outermoston the magnifying side of the 2a lens group and the shape of amagnifying-side surface of the lens element which is disposed secondoutermost from the magnifying side of the 2a lens group;0.1≦N _(IIaN) −N _(IIaP)  (12)10≦V _(IIa1) −V _(IIa2)  (13)0.2≦f/f _(IIa1)≦0.5  (14)0.8≦R _(IIa2) /R _(IIa3)≦1.5  (15) where, N_(IIaN): Mean value ofrefractive indices of the negative lens element which makes up the 2alens group relative to the d line; N_(IIaP): Mean value of refractiveindices of the positive lens elements which make up the 2a lens grouprelative to the d line; V_(IIa1): Abbe number of the lens element whichis disposed outermost on the magnifying side of the lens elements whichmake up the 2a lens group; V_(IIa2): Abbe number of the lens elementwhich is disposed second outermost from the magnifying side of the lenselements which make up the 2a lens group; f_(IIa1): Focal length of thelens element which is disposed outermost on the magnifying side of thelens elements which make up the 2a lens group; R_(IIa2): Radius ofcurvature of a contracting-side surface of the lens element which isdisposed outermost on the magnifying side of the lens elements whichmake up the 2a lens group; and R_(IIa3): Radius of curvature of amagnifying-side surface of the lens element which is disposed secondoutermost from the magnifying side of the lens elements which make upthe 2a lens group.
 5. A lens system as set forth in claim 1, wherein the2b lens group is configured to comprise three lens elements in total,that is, a positive lens element, a negative lens element, and apositive lens element, and wherein the following conditional expressions(16), (17) are satisfied with respect to refractive index and dispersionproperties of a glass material used for each of the lens elements whichmake up the 2b lens group, respectively, the following conditionalexpression (18) is satisfied with respect to a power set for the lenselement which is disposed second outermost from the magnifying side ofthe 2b lens group, and the following conditional expression (19) issatisfied with respect to the shape of a magnifying-side surface and theshape of a contracting-side surface of the lens element which isdisposed outermost on the magnifying side of the 2b lens group;0.15≦N _(IIbN) −N _(IIbP)  (16)20≦V _(IIbP) −V _(IIbN)  (17)−0.75≦f/f _(IIb2)≦−0.25  (18)−6.0≦R _(IIb1) /R _(IIb2)≦−0.8  (19) where, N_(IIbN): Mean value ofrefractive indices of the negative lens element which makes up the 2blens group relative to the d line; N_(IIbP): Mean value of refractiveindices of the positive lens elements which make up the 2b lens grouprelative to the d line; V_(IIbP): Mean value of Abbe numbers of thepositive lens elements which make up the 2b lens group; V_(IIbN): Meanvalue of Abbe numbers of the negative lens element which makes up the 2blens group; f_(IIb2): Focal length of the lens element which lies secondoutermost from the magnifying side of the lens elements which make upthe 2b lens group; R_(IIb1): Radius of curvature of a magnifying-sidesurface of the lens element which is disposed outermost on themagnifying side of the 2b lens group; and R_(IIb2): Radius of curvatureof a contracting-side surface of the lens element which is disposedoutermost on the magnifying side of the 2b lens group.
 6. A lens systemas set forth in claim 1, wherein an adjustment of a focal point of thewhole lens system is implemented by shifting the second lens group in anoptical direction thereof.
 7. A lens system as set forth in claim 2,wherein an adjustment of a focal point of the whole lens system isimplemented by shifting the second lens group in an optical directionthereof.
 8. A lens system as set forth in claim 3, wherein an adjustmentof a focal point of the whole lens system is implemented by shifting thesecond lens group in an optical direction thereof.
 9. A lens system asset forth in claim 4, wherein an adjustment of a focal point of thewhole lens system is implemented by shifting the second lens group in anoptical direction thereof.
 10. A lens system as set forth in claim 5,wherein an adjustment of a focal point of the whole lens system isimplemented by shifting the second lens group in an optical directionthereof.
 11. A lens system as set forth in claim 1, wherein a third lensgroup comprising a single positive lens is provided in the vicinity of alight valve in an airspace between the second lens group and the lightvalve.
 12. A lens system as set forth in claim 2, wherein a third lensgroup comprising a single positive lens is provided in the vicinity of alight valve in an airspace between the second lens group and the lightvalve.
 13. A lens system as set forth in claim 3, wherein a third lensgroup comprising a single positive lens is provided in the vicinity of alight valve in an airspace between the second lens group and the lightvalve.
 14. A lens system as set forth in claim 4, wherein a third lensgroup comprising a single positive lens is provided in the vicinity of alight valve in an airspace between the second lens group and the lightvalve.
 15. A lens system as set forth in claim 5, wherein a third lensgroup comprising a single positive lens is provided in the vicinity of alight valve in an airspace between the second lens group and the lightvalve.
 16. A lens system as set forth in claim 6, wherein a third lensgroup comprising a single positive lens is provided in the vicinity of alight valve in an airspace between the second lens group and the lightvalve.
 17. A projector installing the lens system set forth in claim 1.18. A projector installing the lens system set forth in claim
 2. 19. Aprojector installing the lens system set forth in claim
 3. 20. Aprojector installing the lens system set forth in claim 4.